X-ray sources having as anode a jet of liquid metal is one of the most recent technological paradigms in X-ray generation. Such sources are characterised by their excellent brightness, which brings benefits relating to exposure duration, spatial resolution and new imaging methods, such as phase-contrast imaging.
On the technological level, an X-ray source of this kind includes an electron source and a jet of liquid (preferably a liquid metal with low melting point, such as indium, tin, gallium, lead or bismuth, or an alloy thereof) provided inside a vacuum chamber. More precisely, the electron source may function by the principle of, e.g., cold-field emission, thermal-field emission and thermionic emission. Means for providing the liquid jet may include a heater and/or a cooler, a pressurising means (such as a mechanical pump or a source of chemically inert propellant gas), a nozzle and a receptacle to collect liquid (liquid dump) at the end of the jet. The portion of the liquid jet which is being hit by the electron beam during operation is referred to as the interaction region. The X-ray radiation generated by the interaction between the electron beam and the liquid jet leaves the vacuum chamber through a window. In available X-ray sources, the window consists of a framed thin foil of a suitable material. Requirements on the window material include high X-ray transparency (i.e., low atomic number) and sufficient mechanical strength to separate vacuum from the ambient pressure. Beryllium has widespread use in such windows.
During normal operation of the X-ray source, the window becomes gradually obscured by depositing debris. Not only does the average flux decrease due to absorption of X-rays in such deposited debris, but larger splashes will also manifest themselves as dark spots in the images caused by uneven illumination. The debris mainly consists of material from the liquid jet anode which is transported to the window in gaseous form or as splashes. Debris is chiefly produced by spraying effects at the jet nozzle (especially when it is switched on or off), in the region where the electron beam hits the liquid jet, and at the surface of the liquid contained in the receptacle at the end of the jet. Steps have been taken to reduce the production of debris, cf. granted patent SE 530 094, but there is still a discouraging positive correlation between the output X-ray power and the rate of debris production.